The present invention relates generally to gas turbine engines, and, more specifically, to turbine shrouds therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in turbine stages which power the compressor, and also power an upstream fan in a turbofan gas turbine engine for aircraft applications.
The high pressure turbine (HPT) directly follows the combustor and receives the hottest temperature combustion gases therefrom, and is joined by one driveshaft to the rotor of the compressor for powering thereof during operation. A low pressure turbine (LPT) follows the HPT and includes several rotor stages joined by another driveshaft to the fan typically located forward of the compressor.
During operation, the rotor blades in the HPT extract energy to drive the corresponding rotor blades of the compressor. And, the rotor blades of the LPT extract energy to drive the fan blades conventionally with co-rotation of the turbine blades in the HPT and the LPT rotors.
Each turbine stage includes a turbine stator nozzle that preferentially directs the combustion gases through a cooperating row of turbine rotor blades. An annular turbine shroud surrounds each row of turbine blades and axially bridges the successive turbine nozzles.
The turbine shrouds are spaced closely adjacent to the radially outer tips of the turbine blades for minimizing the radial clearance or gap therebetween for maximizing engine efficiency.
Each turbine shroud is an assembly of components specifically configured for controlling the radial clearance between the shroud and blade tips as the engine operates during transient and steady state conditions. For example, during transient engine operation such as acceleration of the driveshafts during takeoff of the aircraft, the turbine components are heated and radially expand which correspondingly affects the blade tip clearance.
Accordingly, the design of modern turbine shrouds involves complex analysis and the consideration of competing objectives for controlling blade tip clearance while maximizing engine efficiency and life under the various thermal and mechanical stresses experienced by the shroud components.
In one engine design undergoing years of development, a common turbine shroud bridges the HPT and the LPT for certain advantages, but with associated disadvantages as well.
The typical turbine shroud includes a row of shroud segments with each segment having two supporting hooks that engage two corresponding inner hooks in a supporting hanger. The hanger also has two outer hooks supported in a pair of corresponding hooks of a surrounding shroud support. And that shroud support includes two corresponding rings which provide corresponding thermal mass that controls thermal expansion and contraction of the shroud support during transient engine operation.
In the development engine disclosed above, the two different turbine shrouds at the junction of the LPT and the HPT and their associated sets of supporting hook pairs are replaced by a common shroud segment having three supporting hooks which engage three inner hooks of the common hanger, with the common hanger having three outer hooks engaging three corresponding hooks in the shroud support, with the shroud support having three cooperating thermal control rings.
Although this three-hook integrated turbine shroud enjoys certain advantages for increasing engine performance, the mechanical and thermal design thereof is correspondingly more complex.
In particular, maintaining accurate clearance control of the common shroud segment with the two stages of turbine blades is more complex due to the integrated three-hook shroud support.
The three-hook shroud support configuration correspondingly has three different loadpaths therethrough which affect each other in a statically indeterminate manner.
Mechanical design requires detailed analysis of contact points and load transmission through the several sets of cooperating supporting hooks in the shroud assembly, which analysis is used for limiting mechanical and thermal stress during operation for maximizing durability for a correspondingly long useful life.
The three interrelated loadpaths through the multiple sets of cooperating support hooks in the turbine shroud effect redundancy in a statically indeterminate manner which correspondingly increases the variation in mechanical and thermal stress in the shroud components.
Such indeterminate shroud configuration can not only adversely affect the desired clearance control of the engine, but can lead to undesirably shortened shroud life when local stresses are higher than desired.
Accordingly, it is desired to provide an improved dual stage turbine shroud resolving this statically indeterminate problem.